Rational Growth of Symmetric Nanoparticle Arrays into Asymmetric Ones for Surface-Enhanced Raman Scattering Sensing Applications

Different from the extensively studied symmetric nanoparticles, asymmetric plasmonic nanoparticles exhibit unique near-field plasmonic properties suitable for sensing applications, especially after being assembled into ordered arrays. However, reliable fabrication of asymmetric plasmonic nanoparticle arrays remains a challenge, primarily constrained by intrinsic growth habit and assembly behavior. Here, we first assemble the symmetric nanoparticles into an ordered array and then realize directed growth of the array into asymmetric nanoparticle arrays via a ligand patch-protected selective growth strategy. Specifically, we can achieve precise selective overgrowth either in the lateral or vertical direction of the ligand patchy-protected Au nanorice array. Asymmetrically structured Au nanomushroom (NM) array and Au nanosingle-pyramid (NSP) array are fabricated by growth of the Au nanorices along the vertical or horizontal direction. Notably, the surface-enhanced Raman scattering (SERS) activity of the Au NM array is approximately 126 times higher than that of the Au NSP array. The electromagnetic field is drastically amplified at the highly curved gap regions between the “caps” of the Au NMs, which is confirmed by the finite-difference time-domain simulation results. Ultimately, for the Au NM array with an interparticle gap of ∼3 nm, the SERS enhancement factor reach up to 109, fully demonstrating its great potential for sensing and spectroscopy applications.